ABSTRACT. Mussels Mytilus edulis, collected from 26 coastal sites from the Shetland Islands to the Thames estuary and 8 offshore light vessels, were used to monitor changes in environmental quality along the North Sea coastline of the UK (July 1990 and August 1991). The combined measurements of the stress response, scope for growth (SFG), and chemical contaminants in the tissues of mussels were able to detect, quantify and identify some of the major toxicants causing the observed pollution effects. SFG declined from north to south, reflecting both the major inflow of clean water from the North Atlantic via the north of Scotland, and the overall increase in environmental contamination with increasing urbanisation and industrialisation towards the south. There were coastal regions (e.g. Humber-Wash area and the Thames estuary) as well as specific sites (e.g. Ythan, Montrose, Blyth, Teesmouth, Whitby) which showed markedly reduced SFG. Using experimentally derived tissue concentration-response relationships it was shown that at over half the sites the reduced SFG could be entirely explained by the recorded concentrations of contaminants in the tissues. At the majority of sites, a large contribution towards the observed decline in SFG was caused by toxic (mainly polyaromatic) hydrocarbons, largely reflecting urbanisation and shipping activity. In addition, reductions in SFG appear to be partially explained by the accumulation of significant amounts of 'polar organic compounds' and tributyltin. At no sites were metals accumulated to concentrations that could cause a significant reduction in SFG. At those sites with a large 'unexplained component' to the very low SFG values, there was a significant correlation between this 'residual unexplained toxicity' and concentrations of organochlorines In the mussels. More research on the toxicity of these organochlorine compounds to mussels is needed.

Annular flumes were used to quantlfy benthic-pelaqc exchanges in relation to changes in current velocity and sediment biota. Various physical, chemical and biological parameters were determined for 4 sites on the Molenplaat tidal flat in the Westerschelde (The Netherlands) during 2 field campaigns in May-June and August-September 1996. These Pcluded: the mass of sediment eroded, maximum and mean erosion rates, critical erosion velocity (Ucfi,), suspension feeding/biodeposition rate, oxygen and ammonium fluxes, density of macrofauna species, chlorophyll a (chl a), colloidal carbohydrate, and physical properties of the sediment. The study showed marked and statistically significant spatial and temporal differences in the potential for sediment erosion. Sites in the centre of the tidal flat were less erodable than those on the edge. All sites on the tidal flat had a significantly (p < 0.001) lower erosion potential in June (i.e. higher U,,, and lower erosion rates) compared to September.The presence of a well-developed microphytobenthos community in June resulted in a statistically significant relationship between U,,, and chl a/colloidal carbohydrate (r = +0.85, p = 0.01). There was a significant relationship (r = +0.88, p = 0.005) between sediment erodab~lity (mass of sediment eroded and erosion rate) and the density of the clam Macoma balthica, a major bioturbator of the surface sedi m e n t~. Analysis of the data also demonstrated significant relationships between clearance rate and Cerastoderma edule biomass (r = +0.91, p < 0.001), and between sedment chl a/colloidal carbohydrate and C. edule biomass (r = +0.92; p < 0.001), reflecting the influence of suspension feeders on both the water column and the sediment, through their biodeposits. The significant increase in sediment erodability between June and September reflected the shift in the overall balance between the major 'biostabilisers' (microphytobenthos) and 'bio-destabilisers' (bioturbating bivalves). T h s interpretation of field-derived correlations is consistent with previously established relationships and mechanistic understanding derived from controlled flume experiments.

This article synthesises a series of studies concerned with physical, chemical and biological processes involved in sediment dynamics (sedimentation, erosion and mixing) of the Molenplaat tidal flat in the Westerschelde (SW Netherlands). Total sediment accretion rate on the flat (sand to muddy sand) was estimated to be ~2 cm yr -1 , based on 210 Pb and 137 Cs profiles. 7 Be showed maximum activity in the surface sediments during summer, reflecting accretion of fine silt at this time of year, and total vertical mixing of sediment to be in the order of 50 cm 2 yr -1 . The extent to which different physical and biological processes (tidal currents, air exposure, bio-stabilisation, biodeposition and bioturbation) contributed towards sediment dynamics was estimated. A sediment transport model based on physical factors estimated sedimentation rates of 1.2 cm yr -1 , but did not account for tidal or seasonal variation in suspended particulate matter (SPM), wind or effects of spring-neap tidal cycles. When the model was run with an increased critical bed shear stress due to the microphytobenthos, net sedimentation rates increased 2-fold. These higher rates were in closer agreement with the rates derived from the depth profiles of radionuclides for the central region of the tidal flat (2.0 to 2.4 cm yr -1 ). Therefore a significant part of the sedimentation rate (~50%) may be explained by spatial-temporal changes in biological processes, including 'bio-stabilisation' by microphytobenthos, together with the enhanced biodeposition of silt by suspension feeders, and offset by processes of 'bio-destabilisation' by grazers and bioturbators. In the centre of the tidal flat there was a shift from high sediment stability in spring-summer 1996 to low sediment stability in autumn 1997, quantified by a significant reduction in critical erosion velocity of 0.12 to 0.15 m s -1 , and accompanied by a 30-to 50-fold increase in sediment erosion rate. The change was associated with a shift from a tidal flat dominated by benthic diatoms and a low biomass of bioturbating clams (Macoma balthica), to a more erodable sediment with a lower microphytobenthos density and a higher biomass of M. balthica. Vertical mixing of sediment and organic matter, studied using a variety of tracers, was rapid and enhanced by advective water flow at sandy sites and by burrowing polychaetes and bivalves at silty sites. The sediment dynamics and biogeochemistry of tidal flats is dependent on the complex interactions between physical, chemical and biological processes/properties, which include tidal currents, river flows, storm waves, air exposure, dehydration in summer, ice-scour in winter, sediment properties (grain size and composition, organic content, nutrient content, redox balance), stabilisation by biota (algal biofilms, mussel beds, salt marsh) and destabilisation by biota (bioturbating bivalves, scouring around clumps of animals and plants). To date, there have been very few multi-disciplinary studies providing an integrated view of the sedim...

Annular flume experiments and hgh-performance liqud chrom.atography (HPLC) were used to quantitatively and qualitatively measure benthic-pelagc exchange of microphytobenthos from natural sedirnents. Clear spatial and temporal differences in microphytobenthos resuspension in response to stepwse increases in current velocity were observed. Resuspension of chlorophyll a (chl a) from sandy sediments containing low levels of microphytobenthos biomass (<11.5 mg chl a m-2) occurred gradually and continuously over a range of current velocities from 10 to 40 cm S-'. In June, well-developed d~atom mats at siltier sites (>56 mg chl a m-2) displayed strong resistance to erosion at currents <20 to 25 cm S-', above which there was a very rapid increase in the amount of chl a in suspension following the stripping of the algal mat from the sediment surface. In September, when the diatom bloom was over, these sediments were less resistant to erosion and resuspension of microalgae occurred at current velocities above 15 to 20 cm-'. Site 1, situated at the edge of the flat, had a dense algal mat but low sediment stability. Microalgae were readily resuspended because the extracellular polymeric substances (EPS) produced by the migratory diatoms were unable to consolidate during the short emersion period. As a rule more chl a was resuspended from sandy sediments at current velocities 15 and 20 cm-', but above this current veloclty chl a resuspension was greater from silty sediments. Although sandy sites have low biomass in the surface layer, the greater depth of sediment erosion during bedload transport exposes more chl a to the surface. The percentage of sedimentary chl a lost at selected current velocities was estimated, and the implications for carbon supply to the pelagic and benthic systems discussed. Much of the biomass resuspended may be deposited locally, particularly in sandier regions. During the flume experiments it was observed that settling of fine sediment and microalgae was extremely rapid, because it was being biodeposited by suspension-feeding activity, and 'stripped' out of the water column by rapidly sinking suspended particulate matter (SPM). Qualitative changes in suspended material were measured as % chl a (chl a/phaeopigments X 100%) and accessory pigment content. As current velocity increased the relative proportion of phaeopigments increased, which has important implications for benthic suspension feeders feeding in the benthic boundary layer. The findings from the flume experiments have been compared with in situ measurements of current velocity, SPM and chl a.

Our main objective was to investigate the effect of Spartina anglica stems on hydrodynamics, sediment dynamics and their influence on seasonal morphological changes on the upper shore and salt marsh of an intertidal mudflat in the Tavy estuary (southwest England). On an intertidal transect, the greatest seasonal changes in sediment levels occurred at the seaward edge and lower part of the salt marsh, and on the lower shore mudflat, while the smallest changes occurred on the mid-and upper-shore mudflat. Flume studies demonstrated flow attenuation with increasing Spartina stem density, but this was accompanied by an increase in turbulent kinetic energy and bed shear stress (τ 0 ). The critical erosion threshold (τ e ), however, remained relatively constant at 0.12 Pa. Field measurements showed that τ 0 was well below τ e under calm conditions. During periods of windwave activity, when daily mean wind speeds were > 8 m s -1 , τ 0 was greater than τ e in the lower salt marsh for brief periods at the beginning and end of tidal inundation. At the seaward edge of the salt marsh, τ 0 increased to 1.0 Pa and τ 0 was greater than τ e throughout the inundation period, causing significant sediment erosion. Wind-induced wave activity was the major driver of sediment erosion on soft muddy shores and this was enhanced by the interaction with S. anglica stems. The number of days per month with mean daily wind speeds > 8 m s -1 was inversely related to changes in sediment levels at the seaward edge of the salt marsh. During the summer, winds were below this threshold, and there was gradual accretion. Results are discussed in the context of climate changes involving increased storminess leading to increased erosion. We show that the S. anglica salt marsh should not be considered a 'bio-stabiliser' of fine muddy sediment.